Polyoxometalates (POMs) are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge, which is balanced by cations that are external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group IIIa to VIa elements such as phosphorus, antimony, silicon and boron. The framework of polyoxometalates typically comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Due to appropriate cation radius and good π-electron acceptor properties, the framework metal is generally limited to a few elements such as tungsten, molybdenum, vanadium, niobium and tantalum.
In the past, there have been increasing efforts towards the modification of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility, etc.
For example, Kortz et al. report on the palladium(II)-substituted, dimeric, lone pair containing polyanion [Cs2Na(H2O)10Pd3(α-SbW9O33)2]9− (Inorg. Chem. 2004, 43, 3915-3920). This polyanion was synthesized by reacting Pd(CH3COO)2 with [α-SbW9O33]9− in aqueous acidic medium. The square-planar palladium(II) ions are located in the central belt of the sandwich-type structure connecting two (α-SbW9O33) Keggin moieties via bonding to oxygen atoms of the WO6 octahedra. The central belt of this polyoxometalate is completed by two Cs+-ions and one Na+-ion which occupy the vacancies between the palladium centers.
Moreover, Kortz et al. report on the palladium(II)-substituted, lone pair containing polyanion [Cs2Na(H2O)8Pd3(α-AsW9O33)2]9− (Eur. J. Inorg. Chem. 2005, 3034-3041). This polyanion was synthesized by reacting PdCl2 with [α-AsW9O33]9− in aqueous acidic medium and can be considered as the As-analogue of the above mentioned [Cs2Na(H2O)10Pd3(α-SbW9O33)2]9−.
Further, Kortz et al. disclose the synthesis of the dimeric polyanions [(α-XW9O33)2M3(H2O)3]12− (M=Cu2+, Zn2+; X=AsIII, SbIII) (Inorg. Chem. 2001, 40, 4742-4749). Like the above palladium substituted POMs, also these polyanions belong to the class of Hervé-type sandwich POMs, i.e. they comprise two [α-XW9O33] units. The α-Keggin fragments of these polyanions are joined by three equivalent Cu2+ or Zn2+ ions each having one terminal water molecule resulting in square pyramidal coordination geometry. The addenda positions between the three transition metal ions are occupied by three sodium ions leading to a central belt of six metal atoms altering in position. In addition, Kortz et al., Inorg. Chem. 2004, 43, 144-154, describe the preparation of {K7Na[Cu4K2(H2O)6(α-AsW9O33)2].5.5H2O}. This polyanion consists of two (α-XW9O33) units joined by a cyclic arrangement of four Cu2+ and two K+ ions, i.e. the central belt is composed of three adjacent, edge-shared CuO4(H2O) square pyramides and a unique CuO4(H2O) fragment which is separated from the copper triad by two potassium ions.
Furthermore, Yamase et al. describe the synthesis of [(CuCl)6(AsW9O33)2]12− by replacement of a di-lanthanide moiety in [{Eu(H2O)}2(AsW9O33)2]12− with a Cu6 hexagon (Inorg. Chem. 2006, 45, 7698-7704). The hexagon composed of penta-coordinated transition-metal-ions is found to be sandwiched by two [B-α-AsW9O33]9− ligands.
Additional references of interest include:    1. Observation of a Half Step Magnetization in the {Cu3}-Type Triangular Spin Ring, Choi, K.-Y.; Matsuda, Y. H.; Nojiri, H.; Kortz, U.; Hussain, F.; Stowe, A. C.; Ramsey, C.; Dalal N. S. Phys. Rev. Lett. 2006, 96, 107202.    2. The Wheel-Shaped Cu20-Tungstophosphate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− Ion, Mal, S. S.; Kortz, U. Angew. Chem. Int. Ed. 2005, 44, 3777-3780.    3. Synthesis and Structure of the Penta-Copper(II) Substituted Tungstosilicate [Cu5(OH)4(H2O)2(A-a-SiW9O33)2]10−, Bi, L.-H.; Kortz, U. Inorg. Chem. 2004, 43, 7961-7962.    4. Magnetic Properties of Lone Pair Containing, Sandwich-Type Polyoxoanions: A Detailed Study of the Heteroatom Effect Stowe, A. C.; Nellutla, S.; Dalal, N. S.; Kortz, U. Eur. J. Inorg. Chem. 2004, 3792-3797.    5. Sandwich-type Germanotungstates: Structure and Magnetic Properties of the Dimeric Polyoxoanions [M4(H2O)2(GeW9O34)2]12−(M=Mn+, Cu2+, Zn2+, Cd2+). Kortz, U.; Nellutla, S.; Stowe, A. C.; Dalal, N. S.; Rauwald, U.; Danquah, W.; Ravot, D. Inorg. Chem. 2004, 43, 2308-2317.    6. Sandwich-Type Silicotungstates: Structure and Magnetic Properties of the Dimeric Polyoxoanions [{SiM2W9O34(H2O)}2]12− (M=Mn2+, Cu2+, Zn2+). Kortz, U.; Isber, S.; Dickman, M. H.; Ravot, D. Inorg. Chem. 2000, 39, 2915-2922.
However, up to now the known transition metal substituted polyanions have not turned out to be very useful for homogeneous or heterogeneous catalytic applications.
Therefore, it is an object of the present invention to provide transition metal substituted polyoxometalates which are useful as catalysts in homogeneous and heterogeneous oxidation reactions of organic substrates. Furthermore, such transition metal substituted POMs should be easy and reproducible to prepare.
These objects are achieved by polyoxometalates described herein.